Genetics: Heredity and Variation

• Genetics involves the science of heredity and variation.
• The term "Gen" originates from a Greek word meaning to become or grow into something, coined by William Bateson in 1905.
• Genetics is defined as the branch of biology studying heredity and variation.
• Heredity refers to the transmission of traits from parents to offspring.
• Variation refers to differences between parents and offspring, as well as among offspring and individuals in a population.
• Genes are the main determinants of life processes.
• Three interrelated fields exist within genetics: transmission genetics/classical genetics, molecular genetics, and population genetics.

Transmission Genetics (Classical Genetics)

• Encompasses the basic principles of genetics and discusses the relationship between chromosomes and heredity.

Molecular Genetics

• Focuses on understanding the chemical nature of the gene.
• It entails the structure and functions of genes at the molecular level.
• Central Dogma of Molecular Biology processes include replication (DNA to DNA), transcription (DNA to RNA), and translation (RNA to protein).

Population Genetics

• It involves the study of evolution or genetic changes to examine the genetic composition of groups of individuals within the same species, and how this composition changes over time.

Branches of Genetics

Cytogenetics

• Studies the behavior of chromosomes as carriers of genes.

Developmental Genetics

• Focuses on gene regulation during development, including the switching on and off of genes.

Evolutionary Genetics

• Examines the genetic change within and between species.

Biochemical Genetics

• Deals with the roles of enzymes and proteins, products of genes, and the genetic basis of metabolic disorders, such as albinism.

Behavioral Genetics

• Studies inherited behavioral traits and the genetic basis of behavioral disorders like schizophrenia, alcoholism, manic depressive psychosis, and criminality.

Quantitative Genetics

• Concerns itself with the inheritance and expression of quantitative traits, as well as the role of genetic and environmental factors involved.

Applications of Genetics

Agriculture and Biotechnology

• Includes genetically modified organisms or GMOs.
• Products of recombinant DNA technology include Bt corn, cotton, soybean, eggplant, and transgenic papaya with delayed ripening genes, as well as transgenic tobacco with luciferase gene and golden rice rich in provitamin A, along with transgenic fishes and cotton.

Medicine

• It involves the identification of diseases and genetic abnormalities.

Euphenics

• Involves medical and genetic interventions to reduce the impact of defective genotypes.

Legal Applications

• It involves blood type analysis and DNA fingerprinting for paternity testing, disputed parentage, and criminal identification.

Chromosomal Basis of Heredity: Cell Cycle

• The chromosome structure is composed of a chain of nucleosomes.

Levels of Chromatin Packing in The Chromosome

• It includes a folded chromatin fiber acting as carrier of genes.
• The chromosome consists of chromosome arms, centromere, and telomeres with a protein scaffold.

Chromosome Structure

• It consists of sister chromatids, centromere, chromosome arms, and telomere.

Genome

• It represents the complete set of genes or chromosomes from a male or female parent.
• The basic chromosome number is represented as X=n in a true diploid organism.
• Somatic chromosome number is the diploid number (2n).
• Gametic chromosome number is the haploid number (n).

Mitosis

• It results in two identical cells via true equational division, going through the stages like prophase and metaphase.
• In prophase the chromosomes are visibly composed of sister chromatids.
• In metaphase, chromosomes align at the equatorial plane.
• During anaphase, sister chromatids separate and move to opposite poles.
• In telophase, chromosomes are regrouped at the poles, after cytokinesis, where two cells are formed.

Meiosis

• Meiosis involves reductional division, resulting in haploid cells that are distinct.
• Stages of meiosis include Meiosis I (PI, MI, AI, TI) and Meiosis II (PII, MII, AII, TII).

Prophase I in Meiosis

• It consists of five substages called Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
• Leptotene is the thin thread stage.
• Zygotene is the homologous pairing stage that forms bivalents (II), where chiasmata holds II together.
• Pachytene is the crossing-over stage that forms chiasma, leading to genetic recombination.
• Diplotene involves separation starting at the centromere and terminalization of chiasmata.
• Diakinesis is where II are evenly distributed for the best stage to establish chromosome number.

Meiosis I stages

• The metaphase I, aligns II at the equatorial plane
• During anaphase I the separation of II into univalent (I) accounts for reductional division.
• During telophase I the chromosomes regroup at the opposite poles, and two haploid cells are formed.

Meiosis II

• It involves the separation of sister chromatids during anaphase II.
• During telophase II, four haploid cells are formed while still using stage prophase and metaphase II.

Life Cycles

• Gametic or Terminal has an alternate name called the diploid-dominant life cycle.
• Zygotic or Initial alternates as the haploid-dominant life cycle.
• Sporic or Intermediary alternates as alternation of generations.

Definition of Terms

• A gene represents an inherited factor on the chromosome, responsible for a trait.
• A locus is the location of a gene on a chromosome.
• Genotype refers to the genetic constitution of an individual.
• Allele refers to alternative forms of a gene.
• Phenotype refers to the physical, physiological, biochemical, and behavioral traits of an individual, which are determined by its genotype and interaction with the environment.
• Dominant allele exerts full effect despite the presence of another allele of the same gene.
• Recessive allele is not expressed when another (dominant) allele of the same gene is present.
• Homozygous individuals have two copies of the same allele of a gene.
• Heterozygous individuals contains two different alleles of the same gene.

Hybridization

• Hybridization is the cross between two individuals with constrasting traits.
• The F1 or First Filial Generation is the first generation produced after mating between parents that are homozygous for different alleles.
• The F2 Generation (or Second Filial Generation) refers to self-fertilization or sib-mating of F1 individuals.
• Backcrossing refers to a heterozygote being crossed with one of its parents.

Pre-Mendelian Heredity

• States that characteristics are a mixture of both parents.

Gregor Mendel

• Hypothesized that blending does not adequality explain the offsprings can only resemble one parent.
• Mendel used self-pollinated plants to test it.

Mendel's Laws

• The Law of Segregation says alleles in a gene pair separate during meiosis.
• The Law of Independent Assortment states the alleles of different gene pairs and separate independently during meiosis and randomly combine.
• The product law of probabilities stats two independent events can and will occur.

Correlations Between Chromosomes and Factors

• Chromosomes exist in pairs and the allelic factos come from the parents.
• Homologous chromosomes separate during anaphase I.
• Fertilization restores the diploid chromosome number.
• Alleles of a gene pair up again.

Types of Dominance

• Complete Dominance means one dominant allele is enough to express the dominant trait.
• Incomplete Dominance is when the F1 phenotype is intermediate.
• Overdominance means the heterozygote is more superior than the homozygotes.
• Co-Dominance states that both alleles are only fully expressed in the heterozygote.

Gene Interactions

• Non-allelic interaction occurs when two or more genes determine a single phenotype.
• Novel Phenotypes involves two genes interacting to create a new phenotype.
• Recessive Epistasis involves a homozygous recessive gene that hides the effect of another gene.
• Dominant allele masks the expression of another gene.
• Complementary Gene Action involves either of the genes, where a homozygous recessive is epistatic to the other.
• Either of the dominant genes are epistatic to the other while following Duplicate Gene Action.

Environmental Influence on Gene Expression

• Phenotype equals genotype plus the environment.
• Twin Studies involve identical and fraternal twins to assess genetic vs. environmental influence.
• Penetrance describes the amount of individuals that show expected trait with complete 100% to no 0% showing.
• Expressivity is the degree of trait expression.

Pleiotropy

• One gene that affects mutiple traits.

Phenocopy

• When environmental create traits that mimic genetic traits.

Twin Studies in Genetics

• Identical Twins have the same genetic makeup.
• Fraternal Twins have different genetic makeups.
• Concordance means that both twins share a twin.
• Discordance Only one twin show a trait.